The present invention relates to a tracking defect preventing circuit in a recording/reproducing apparatus using an optical recording medium, which prevents such a failure in tracking servo control as is caused by a scratch, stain or dust on a recording surface of the recording medium. More specifically, an embodiment of the invention is adapted for a reader/writer of an "optical card".
In a recording/reproducing apparatus of the optical card, for example, focusing control and tracking control of a laser beam which illuminates a recording surface of the optical card are performed as shown in FIG. 1. The focusing and tracking control shown in FIG. 1 is called a "three beams method", where a main beam spot MS is used for the focusing control and sub-beam spots SS.sub.1 and SS.sub.2 are used for the tracking control. Using these sub-beam spots SS.sub.1 and SS.sub.2, corresponding tracking error signals E and F are produced. FIG. 1 assumes the case where some foreign substance FS such as a scratch, stain, etc. are placed on the recording surface of the card C in such a manner that the foreign substance FS spans both of at least one of guide tracks GT (dark area) and an information-recording area L (light area). Even in the case where the card C is moved in a direction indicated by an arrow with the two sub-beam spots SS.sub.1 and SS.sub.2 being located on the two guide tracks GT with the same condition (e.g., both spots making just landing on the respective guide tracks), only the front spot SS.sub.1 firstly contacts with the substance FS, causing abrupt occurrence of the imbalance between the tracking error signals E and F. According to this imbalance in the tracking error signals, a difference error signal which is produced by an error signal amplifier is suddenly increased to a large value, and therefore an optical pickup, which is driven by an actuator, is removed away from the track to be traced.
In a CD (Compact Disk) player, the above-described problem is solved by delaying the tracking error signal corresponding to the front sub-beam spot. However, this method is not applicable to the optical card because of the following two reasons. While in the CD player linear moving velocity of a disk is about 1.2 m/sec, in the optical card moving velocity of the card is extremely low, that is, 68.3 mm/sec at a writing (recording) operation and 410 mm/sec at a reading (reproducing) operation. As a result, if the above delaying method is applied to the optical card, a frequency range of the delaying operation becomes located within that of tracking servo and hence the tracking control cannot be performed. The other reason is that a linear shuttle movement is employed in the optical card.